The invention relates to a transducer and, more particularly, to a transducer for detecting a mechanical displacement by an electrostatic capacitance and converting it into an electric signal.
Conventionally, as a transducer for converting a displacement by a mechanical pressure into an electric signal, there is a pressure sensor of an electrostatic capacitance detecting type disclosed in JP-A-9-257618, for example. FIGS. 1-3 are diagrams showing a structure, a manufacturing process and a capacitance-to-voltage converting circuit of the transducer. FIG. 1 is a plan view of the pressure sensor and is a diagram mainly showing an arrangement of electrodes. FIGS. 2A-2G are cross sectional views taken along the line A-A' in FIG. 1 which are shown in accordance with the order of manufacturing steps. The pressure sensor is constructed by a fixed electrode formed on a frontside surface of a substrate; a pressure reference chamber provided over the fixed electrode; an insulating diaphragm formed so as to cover the pressure reference chamber; and a moving electrode as a conductive film of the surface of the insulating diaphragm.
FIG. 3 is a block diagram showing a capacitance-to-voltage converting circuit which detects the electrostatic capacitance of the fixed electrode and the moving electrode which are formed as mentioned above. The capacitance-to-voltage converting circuit is constructed by a switched capacitor circuit. The capacitance-to-voltage converting circuit shown in FIG. 3 has a reference capacitor to absorb a distribution of electrostatic capacitance values due to a dimensional variation in the manufacturing steps and a fluctuation of the electrostatic capacitance values which is caused by a dimensional change due to a temperature fluctuation or the like. The pressure sensor shown in FIG. 1 does not have a construction including such a reference capacitor. The reference capacitor can be easily constructed by adjacently forming a capacitor in which a sacrificial layer is not etched.
The outline of the manufacturing steps of the conventional pressure sensor will now be described with reference to FIGS. 2A-2G.
By diffusing the impurities into the frontside surface (surface on the upper side in the diagram) of a substrate 100 made of monocrystalline silicon, a fixed electrode 111, a fixed electrode lead 112 and a fixed electrode lower connecting terminal 113 all of which have conductivity are formed. After that, a first insulating layer 120 is deposited onto the frontside surface of the substrate 100 (refer to FIG. 2A).
Subsequently, a sacrificial layer 140 is deposited onto the first insulating layer 120 (refer to FIG. 2B). After that, as shown in FIG. 2C, a first insulating diaphragm layer 150 is deposited onto the first insulating layer 120 and the sacrificial layer 140. After that, a first conductive layer 110 is deposited onto the first insulating diaphragm layer 150. The first conductive layer 110 is etched while leaving the portion of a moving electrode 161 and the portions of a moving electrode lead 82 and moving electrode lower connecting terminal 163 which are used for the electrical connection of the moving electrode. After that, as shown in FIG. 2D, a second insulating diaphragm layer 170 is deposited onto the first insulating diaphragm layer 150 and the first conductive layer 110, and then an etching liquid feeding hole 10 which penetrates the second insulating diaphragm layer 170 and the first insulating diaphragm layer 150 and reaches the sacrificial layer 140 is formed.
Subsequently, by feeding the etching liquid for isotropically etching the sacrificial layer 140 from the etching liquid feeding hole 10, the sacrificial layer 140 is etched. Thus, as shown in FIG. 2E, a pressure reference chamber 20 is formed between the first insulating layer 120 and the first insulating diaphragm layer 150. Further, a moving electrode connecting hole 11 which penetrates the second insulating diaphragm layer 170 and reaches the moving electrode lower connecting terminal 163, and a fixed electrode connecting hole 132 which penetrates the second insulating diaphragm layer 170, the first insulating diaphragm layer 150 and the first insulating layer 120 and reaches the fixed electrode lower connecting terminal 113, are formed.
Subsequently, a conductive layer is deposited onto the frontside surface. As shown in FIG. 2F, the conductive layer is etched while leaving the portion of a moving electrode output terminal 181 connected to the moving electrode lower connecting terminal 163 through the moving electrode connecting hole 11 and the portion of a fixed electrode output terminal 182 connected to the fixed electrode lower connecting terminal 113 through the fixed electrode connecting hole 132. After that, a sealing material layer is deposited onto the second insulating diaphragm layer 170 so as to seal the etching liquid feeding hole 10. As shown in FIG. 2G, the sealing material layer is etched while leaving the portion of a sealing cap 30 near the etching liquid feeding hole 10.
As mentioned above, the conventional pressure sensor is constructed by including the substrate in which the fixed electrode is formed on the frontside surface; the first insulating diaphragm layer which partitions and forms the pressure reference chamber so as to be away from the frontside surface by only a predetermined distance; the moving electrode formed onto the first insulating diaphragm layer by the conductive layer; the second insulating diaphragm layer deposited so as to cover the moving electrode; the opening (etching liquid feeding hole) which penetrates the second insulating diaphragm layer and the first insulating diaphragm layer and reaches the pressure reference chamber; and the sealing material (sealing cap) for sealing the opening, thereby sealing the pressure reference chamber.
The diaphragm comprising the first and second insulating diaphragm layers of the conventional pressure sensor is deformed in accordance with an ambient pressure. That is, a force in the direction adapted to widen the distance between the diaphragm and the fixed electrode by a pressure from the inside of the pressure reference chamber and a force in the direction adapted to narrow the distance between the diaphragm and the fixed electrode to approach by a pressure that is applied from the outside are applied to the diaphragm, so that the diaphragm is deformed by only an amount corresponding to a difference between those forces. Thus, an electrostatic capacitance of a capacitor constructed by the fixed electrode and the moving electrode formed on the diaphragm shows a value according to the deformation of the diaphragm. By measuring the electrostatic capacitance value, the difference between the pressure in the pressure reference chamber and the pressure applied to the sensor can be known. By setting the pressure in the pressure reference chamber to a value that is enough smaller than a pressure measuring range of the sensor, the sensor can be formed as an absolute pressure measuring type.
The electrostatic capacitance value between the moving electrode and the fixed electrode of the conventional pressure sensor is measured by the capacitance-to-voltage converting circuit as shown in FIG. 3. Switches SWr, SWx, SWf and SWo of the voltage control type are connected to the upper contact (black circle) side when a clock voltage source Vck is at the high (H) level, and are connected to the lower contact (white circle) side when the clock voltage source Vck is at the low (L) level. The switches SWr, SWx, SWf and SWo are constructed so as to operate in an interlocking relation and to be alternately connected to the upper contact side and the lower contact side. Reference character Vb denotes a bias voltage source; Al an operational amplifier; Cf a feedback capacitor; Co an output capacitor for the ripple smoothing; Eo an output voltage; Cx a capacitor constructed by the moving electrode 161 and the fixed electrode 111; Cr a capacitor constructed by a reference electrode 51 and the fixed electrode 111; and Csx and Csr capacitors each constructed by the fixed electrode 111 and the substrate 100. Since an open loop gain of the operational amplifier Al is very large, an electric potential at a (-) input terminal of the operational amplifier is substantially equal to that at a (+) input terminal in consideration of the circuit construction. Thus, an electric potential of the fixed electrode output terminal 182 is set to the ground potential.
The specific operation of the capacitance-to-voltage converting circuit shown in FIG. 3 will now be described. The clock voltage source Vck is changed from the low level to the high level, and the switches SWr, SWx, SWf and SWo are connected to the upper contact side. At a time when the movement of charges is completed after that, since both of the electric potentials of the reference electrode 51 and fixed electrode 111 of the capacitor Cr are set to the ground potential, an amount of charges stored (or charged) in the capacitor Cr is equal to "0". Since an electric potential of the moving electrode 161 of the capacitor Cx is equal to a voltage (Vb) of the voltage source Vb and an electric potential of the fixed electrode 111 of the capacitor Cx is equal to the ground potential, the accumulation charges Qx in the capacitor Cx are equal to Cx.multidot.Vb. Since two terminals of the capacitor Cf are short-circuited by the switch SWf, the electric potentials at those terminals are equal to the electric potential of the fixed electrode output terminal 182, so that the accumulation charges of the capacitor Cf are equal to "0". Since the capacitor Co is disconnected from the output of the operational amplifier Al, the output voltage Eo of the capacitance-to-voltage converting circuit is holding the electric potential according to the charges stored at the last time when the clock voltage source Vck is at the low level.
Subsequently, the clock voltage source Vck is changed from the high level to the low level and the switches SWr, SWx, SWf and SWo are connected to the lower contact side. At a time when the movement of charges is completed after that, since the voltage across the capacitor Cx is equal to "0", the (+) charges are moved from the (-) input terminal of the operational amplifier Al toward the fixed electrode 111 in the direction adapted to set the accumulation charges to "0". Since the voltage across the capacitor Cr is equal to Vb, the (+) charges are moved from the fixed electrode 111 toward the (-) input terminal of the operational amplifier Al in the direction adapted to set the zero charges to the accumulation charges Qr (=Cr.multidot.Vb). Those charges are stored into the capacitor Cf since the switch SWf is open. The magnitude of the charges is expressed by the following equation. EQU Qr-Qx=Cr.multidot.Vb-Cx.multidot.Vb=Vb(Cr-Cx)
Therefore, a voltage Vcf across the capacitor Cf is expressed by the following equation. EQU Vcf=Vb(Cr-Cx)/Cf
Since one end of the capacitor Cf connected to the (-) input terminal of the operational amplifier Al is set to the zero potential, the polarity of the output voltage Eo of the capacitance-to-voltage converting circuit is inverted and the voltage Eo is expressed as follows. EQU Eo=Vb(Cx-Cr)/Cf
The capacitor Co stores the charges corresponding to this potential.
As mentioned above, for a period of time during which the clock voltage source Vck is at the high level, the charges are stored in the capacitor Cx, the charges in the capacitor Cf are set to "0", and the value for a period of time during which the clock voltage source Vck just before is at the low level is outputted as the output voltage Eo. For a period of time during which the clock voltage source Vck is at the low level, the charges in the capacitor Cx are set to "0", the charges are stored in the capacitor Cf, and the voltage expressed by Eo=Vb(Cx-Cr)/Cf is outputted as the output voltage Eo. Since the output voltage Eo is proportional to the capacitance of the capacitor Cx, it is inversely proportional to the reciprocal of the distance between the moving electrode 161 and the fixed electrode 111 which are formed as shown in FIGS. 1 and 2A-2G. The distance is proportional to the pressure applied to the diaphragm comprising the first insulating diaphragm layer 150, the first conductive layer 110 and the second insulating diaphragm layer 170. Therefore, the electrostatic capacitance detecting type pressure sensor from which the output voltage Eo that is inversely proportional to the reciprocal of the pressure applied to the diaphragm is outputted can be constructed.
However, in the apparatus of the electrostatic capacitance detecting type like the conventional pressure sensor mentioned above, since a micro capacitance of a capacitor constructed by the fixed electrode and the moving electrode is detected at a high input impedance, the electric lines of force from the outside of the apparatus drop to the fixed electrode, causing noises. The distance between the fixed electrode and the conductive layer existing in the lower layer of the fixed electrode is short, their parasitic electrostatic capacitance is larger than the capacitance to be detected and exerts a large influence on the capacitance-to-voltage converting circuit. In the foregoing conventional pressure sensor, the construction of the switched capacitor circuit is devised, thereby making it difficult to be influenced from the parasitic capacitance with the substrate. However, a circuit for generating a clock is necessary and the clock exerts an influence on another circuit.
Further, in the switched capacitor circuit with the construction like the conventional pressure sensor mentioned above, only the output voltage concerned with the reciprocal of the displacement of the diaphragm is obtained and an output directly regarding the displacement of the diaphragm cannot be obtained. As a converting system which has a simple construction and can obtain an output that is directly concerned with the displacement of the diaphragm, for example, there is a system for storing the constant charges into the fixed electrode and converting. However, according to the construction like the conventional pressure sensor mentioned above in which the parasitic capacitance between the fixed electrode and the substrate is large, the sensitivity is low and it is difficult to obtain the practical sensitivity.
In the capacitance-to-voltage converting circuit shown in FIG. 3, a device to shield the electric lines of force from the frontside surface of the substrate is made. That is, the voltage source or the ground is always connected to the moving electrode 161 which faces the frontside surface side of the pressure sensor and the reference electrode 51 although they are switched by a switch. Thus, even if the electric lines of force from an external noise source drop to the pressure sensor, since the charges flow to the ground through the voltage source or directly, they do not stray into the fixed electrode and there is a shielding effect against the electrostatic noises. However, no consideration is made to a device to shield against the electric lines of force which enter from the backside surface (surface on the lower side shown in FIGS. 2A-2G) of the substrate 100. This is because the general pressure sensor solves such a problem by a method whereby a conductive material also serving as a shield is used for a pressure vessel to enclose the pressure sensor. However, in case of constructing a smaller transducer, this solving measure becomes an obstacle.